1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) using ferroelectric liquid crystal as a display medium, and a method for producing the same.
2. Description of the Related Art
An LCD of the type using the ferroelectric liquid crystal (FLC) generally utilizes a so-called chiral smectic C phase (Sc*), one kind of liquid crystal phases. In this liquid crystal phase, the orientation of the liquid crystal molecules is a helical one in a bulk state. However, if the liquid crystal material in such a phase is injected into a liquid crystal cell in which a distance between a pair of substrates is shorter than the helical pitch of the liquid crystal material, then the helical structure of the liquid crystal material is deformed. As a result, as shown in FIGS. 9(a) and 9(b), a plurality of smectic layers 101 are stacked in parallel, and the liquid crystal molecules 100 are tilted with respect to the smectic layers 101.
In this liquid crystal phase, bistable states as shown in FIGS. 9(a) and 9(b) are caused upon the application of an electric field. This is because the ferroelectric liquid crystal molecule has a spontaneous polarization (Ps) in a direction vertical to the paper sheet, and therefore, if applying an electric field (E) in the direction of the polarization, then the liquid crystal molecules 100 are reoriented so that the direction of the spontaneous polarization is aligned with the direction of the electric field (E).
By sandwiching the liquid crystal cell including such a liquid crystal material with a pair of polarizing plates, i.e., a polarizer and an analyzer, two kinds of displays, i.e., a display in a bright state shown in FIG. 9(a) and a display in a dark state shown in FIG. 9(b), may be selectively conducted (cf. N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980)).
Since the two kinds of states shown in FIGS. 9(a) and 9(b) are switched because of a direct interaction between an electric field and a spontaneous polarization, if the application direction of the electric field is changed, then a fast response may be realized in the order of microseconds. In addition, the ferroelectric liquid crystal has a so-called memory characteristic. That is to say, even after the applied electric field is removed, the ferroelectric liquid crystal molecules maintain the orientation of the state prior to removing the electric field. Therefore, by utilizing the fast response characteristic and the memory characteristic, the content to be displayed may be written into each scanning line at a high speed, thereby realizing a large capacity display by a simple matrix driving.
FIG. 10A shows a fundamental structure of an FLCD using the ferroelectric liquid crystal. This FLCD includes: a pair of glass substrates 101; a pair of electrodes 102 made of indium tin oxide (ITO) formed on the pair of substrates 101, respectively; a pair of insulating films 103 formed on the electrodes 102; and a pair of alignment films 104 formed on the insulating films 103. The alignment films 104 are generally made of a polymer material such as polyimide, and the surface of the films 104 are subjected to a rubbing treatment. The pair of substrates 101 having the above-described electrodes and films thereon are attached to each other so that the cell gap is approximately 1.5 .mu.m. Then, a liquid crystal material is injected into the gap between the pair of substrates 101 so as to form a liquid crystal layer 105, and the peripheral portions thereof are sealed with a sealing member 106. Thereafter, a pair of polarizing plates, e.g., an analyzer 107 on the upper side and a polarizer 108 on the other lower side, are placed so as to sandwich the liquid crystal cell thus constructed, and drivers (not shown) are connected to the respective electrodes 102.
The ferroelectric liquid crystal display device having the above-described construction as shown in FIG. 10A is almost the same as a simple-matrix type liquid crystal display device as shown in FIG. 10B except that the cell gap of the FLCD of FIG. 10A is as small as 1.5 .mu.m, and that the liquid crystal layer 105 of the FLCD of FIG. 10A is constituted by the ferroelectric liquid crystal material. In these FIGS. 10A and 10B, the same reference numerals denote the same components. In FIG. 10B, the reference numeral 105a denotes a liquid crystal layer constituted by a non-ferroelectric liquid crystal material.
The above-mentioned FLCD using a ferroelectric liquid crystal material has a problem in that the FLCD has a poor resistance to a mechanical shock or a pressure (cf. N. Wakita et al., Abstr. 4th International Conference on Ferroelectric Liquid Crystals, 367 (1993)). The cause of the problem is understood as follows. If applying a pressure or a mechanical shock to the FLCD, the liquid crystal molecules in the ferroelectric liquid crystal layer are forced to flow, so that an initial orientation of the liquid crystal molecule is destroyed never to be naturally recovered.
In order to increase the shock resistance, it is considered necessary to prevent a flow of the liquid crystal molecules from being generated inside the ferroelectric liquid crystal cell because of the shock applied thereto. According to an exemplary method, as shown in FIG. 11, walls 110 are provided between the pair of substrates 101. In FIG. 11, the same components as those in FIGS. 10A and 10B are denoted by the same reference numerals. Various techniques are employed for producing the walls 110. For example, Japanese Laid-Open Patent Publication No. 59-201021 discloses a technique for producing the walls with spacers. Japanese Laid-Open Patent Publication No. 3-192334 discloses a technique for producing the walls by using partition members. Moreover, Japanese Laid-Open Patent Publication No. 6-301015 discloses a technique for forming photopolymerizable polymer walls in a liquid crystal layer by injecting a mixture containing a photopolymerizable resin and a liquid crystal material and then by irradiating the mixture through a photomask with UV rays.
However, if the walls are formed inside a ferroelectric liquid crystal layer, orientational defects are likely to occur in the vicinity of the walls, so that the resulting display quality is degraded because of the effects of the orientational defects.
In order to solve such a problem, as shown in FIG. 12, it is possible to cover the disclination portions in the vicinity of the walls 110 with a light-shielding layer 111. For example, Japanese Laid-Open Patent Publication Nos. 6-347765 and 6-308500 disclose a technique for providing a light-shielding layer for a ferroelectric liquid crystal display device in which the walls are formed. But, in these cases the area of display portions 112, which are not covered with the light-shielding film 111, for contributing to display is reduced by providing the light-shielding film 111, so that the aperture ratio is reduced and the resulting display becomes disadvantageously dark. Also, these patent publications do not disclose a method for controlling the positions where the orientational defects occur.